Rob Portman isn't a bomb thrower. But his pitch last week for an amendment cracking down on countries that artificially weaken their currencies threatened to blow up President Barack Obama's trade agenda, and exposed a deep split in approaches among Republicans who face the voters in 2016.

A trio of Rust Belt Senate Republicans — Ohio's Portman, Ron Johnson of Wisconsin and Patrick J. Toomey of Pennsylvania — ended up voting for Trade Promotion Authority, backing Obama, Republican leadership and a top priority of the U.S. Chamber of Commerce.

But that came only after Portman's currency gambit, which could help blunt Democratic attacks next year.

In states where manufacturing is a major part of the economy — and the scars from globalization dot the countryside — trade bills can be particularly toxic.

And perhaps no one running for re-election is more vulnerable on the issue than Portman, who served as U.S. trade representative under President George W. Bush and voted for the North American Free Trade Agreement as a House member. But Portman’s calculation is that Ohioans want trade as long as it’s fair.

“What I’m hearing when I’m back home is that people want more exports because they know that results in not just more jobs, but better paying jobs,” Portman said. “And we’re a big exporting state. We have an [agriculture] community that is dependent on exports for prices.”

Portman, along with Democratic co-sponsors, pushed for two amendments to the recent trade bill that could provide the political cover needed for his vote in favor of last week’s trade legislation.

One — viewed by many as a poison pill — was the currency manipulation amendment, which drew a veto threat. The other — which paired him with Ohio Democrat Sherrod Brown — would have strengthened enforcement against foreign producers using “unfair ” trade practices. The latter never received a vote.

Brown declined to speculate on the race and he said he was happy to work with Portman on the two amendments — but said he "wished" Portman would have voted no on the underlying Trade Promotion Authority package. Portman voted yes after he got assurances from Ways and Means Chairman Paul D. Ryan, R-Wis., of inclusion of a provision sought by the steel industry in another trade-related bill.

Democrats say their polling shows trade could be a powerful issue in their favor, and the Ohio Democratic Party has sent out near-daily press releases attacking the senator on the issue. A Quinnipiac poll in April found 44 percent of Ohioans don’t know enough about Portman to say whether they have a favorable or unfavorable opinion of him, suggesting Democrats could have a lot of leeway to define the incumbent.

Portman has been aggressive in trying to refocus the debate on the benefits of trade and his amendments.

“I’m at odds with [the Republican Party] on a few things, and that’s OK,” Portman said. “I’m going to fight for Ohio; that’s my job here. It’s not to be doing what my party wants me to do, it’s to do what’s best for our state. And clearly, that’s what’s best for our state.”

Democrats, meanwhile, have accused him of a contortionist act to insulate himself from his record.

“I would characterize his behavior in terms of his emphasizing the China currency issue and so on as not unlike someone who finds religion in a foxhole,” said former Democratic Ohio Gov. Ted Strickland, Portman's 2016 rival.

NAFTA, the 1994 trade agreement, is also playing a primary role in Ohio's trade politics, with Portman hitting Strickland’s vote in Congress against the measure as a vote against exports, while Strickland derides the trade bill as a job killer.

Portman has begun running Web ads attacking Strickland for the loss of jobs in Ohio during his governorship, calling the former governor a hypocrite on the subject of trade for various votes in Congress and messaging while governor.

“From the lost jobs, to his ineffective management, to his blatant hypocrisy Strickland gave Ohio families an era they can’t afford to repeat,” one of the Portman emails read.

In Wisconsin, Republicans say Johnson — who, like Toomey, voted against the currency manipulation language both as part of a standalone bill a few weeks ago and last week’s amendment — will defend his votes in terms of his personal story. Before he was elected to the Senate, he was the CEO of a Wisconsin plastics company.

Johnson, said his campaign manager, Betsy Ankney, “is a manufacturer whose career was based on creating jobs in Wisconsin through trade” and who “used trade to create jobs for Wisconsinites.”

Johnson noted his experience as an exporter in an interview with CQ Roll Call, saying the currency amendment was not only a poison pill, but steeped in hypocrisy. In Johnson’s estimation, countries rarely manipulate their currency for long stretches because market forces eventually take over, but while they do, American consumers benefit from cheaper foreign products.

“In terms of currency manipulation, the U.S. is the pot calling the kettle black,” Johnson said. “I just don’t believe that any country long-term can manipulate their currency. How many times have you heard people want a strong dollar? That means when the other currencies are weaker, that allows us to buy their goods at a lot cheaper price. That benefits U.S. consumers.”

The influential conservative group, Club for Growth, added the currency manipulation vote to their annual scorecard, labeling the amendment protectionism. But the Senate offices of both Toomey, the former head of the group, and Johnson denied Club scoring influenced them.

Immediately after the standalone vote a few weeks ago on currency language, the Democratic Senatorial Campaign Committee sent an attack email against Toomey for his "no" vote.

“Manufacturers and steelworkers are a vital part of Pennsylvania’s economy, and it’s shocking that Pat Toomey chose to vote against a bill that aims to protect them,” Sadie Weiner, the DSCC's national press secretary, said in the email. “Pennsylvanians deserve a Senator who puts his own state’s interests first and works to keep the playing field level for American manufacturers.”

For Toomey, voting on the trade bill is a matter of principle, having staked his career on being a principled conservative and supporting free markets.

“I’m voting for what I think is the best policy for Pennsylvania,” Toomey said. “And that’s to vote no.”